Building stone assessment : comparison of stone samples from Kisimul Castle (Barra) and the island of Stulaigh (by South Uist)

BGS has been asked by Jamie MacPherson, of Historic Environment Scotland, to perform a Building Stone Assessment on two samples of stone from the Outer Hebrides, both provided by the client.  Sample from Kisimul Castle on Barra (OS Grid Reference [NL 6651 9796]): a tabular block of greyish-green metamorphic rock c. 20x15x5cm in size, which is understood to be typical of the main decorative stone used in the castle dressings. The BGS sample number ED11450 has been assigned. The stone forming this sample was identified as mylonite in a previous BGS report1.  Sample from the island of Stulaigh, which lies off the east coast of South Uist. A grid reference for the sample has not been provided; Stulaigh is at OS Grid Reference [NF 83 23]). The sample, which is a tabular block of greyish-green metamorphic rock c. 40x12x5cm in size, is assumed to come from a bedrock outcrop on the island. The BGS sample number ED11483 has been assigned. The objectives of this Building Stone Assessment are to:  provide detailed petrographic descriptions of both samples, to establish the range and character of their intrinsic properties; and  compare the geological character of the samples to assess whether Stulaigh could have been the source of the dressing stone used in Kisimul Castle. A brief summary of key features of the bedrock geology of Barra and South Uist is presented in section 2, to provide context. A petrographic description of each sample, with accompanying photographs, is presented in Appendix 1. The geological character of the samples is compared in section 3, and conclusions are presented in section 4.